The eval broker's `Cancelable` method used by the cancelable eval reaper mutates
the slice of cancelable evals by removing a batch at a time from the slice. But
this method unsafely uses a read lock despite this mutation. Under normal
workloads this is likely to be safe but when the eval broker is under the heavy
load this feature is intended to fix, we're likely to have a race
condition. Switch this to a write lock, like the other locks that mutate the
eval broker state.
This changeset also adjusts the timeout to allow poorly-sized Actions runners
more time to schedule the appropriate goroutines. The test has also been updated
to use `shoenig/test/wait` so we can have sensible reporting of the results
rather than just a timeout error when things go wrong.
